The invention relates to a method of manufacturing ferroelectric layers of bismuth titanate Bi.sub.4 Ti.sub.3 O.sub.12 on a substrate.
Thin ferroelectric layers are important for many electrical and optical applications such as, for example, the manufacture of memories, displays or capacitors. It is known that for such layers ferroelectric compounds having the perovskite structure are used. For ferroelectric memories use is made of, for example, lead-containing perovskite materials. However, these materials have the disadvantage that in their manufacture lead oxide is evaporated, in particular at high temperatures, as a result of which it is often very difficult to control the composition. Lead-containing perovskites such as, for example, PbTi.sub.0,5 Zr.sub.0,5 O.sub.3 exhibit high dielectric constant values .epsilon. of approximately 1500. However, materials having a high dielectric constant can be used for ferroelectric memories only under certain conditions.
A ferroelectric material system which, in the manufacture, exhibits no lead-evaporation problems and which, in addition, has a relatively low dielectric constant (.epsilon.&lt;150) is bismuth titanate Bi.sub.4 Ti.sub.3 O.sub.12. A further distinctive feature of this material is that on switching the ferroelectric material, a rotation through only 9.degree. takes place along the c-axis of the polarization vector.
Up to now thin bismuth titanate layers have been manufactured by means of high-frequency cathode-sputtering methods in which MgO substrates are used. However, cathode-sputtering methods have the disadvantage that an accurate control of the stoichiometry of the layer composition in a multicomponent system is very difficult. In addition, carrying out experiments with these methods involves high costs because they must be conducted in a high vacuum. The deposition of multicomponent systems further requires a very accurate process control. An additional disadvantage of cathode-sputtering methods is that they are not suitable for continuous production processes. The known layers manufactured by means of cathode-sputtering methods had a thickness in the range from 12 to 32 .mu.m. At such layer thicknesses, however, the ferroelectric layers cannot be used as memories.